The present invention relates to a running control apparatus for engine-driven vehicles, and particularly to a vehicle running control apparatus in which an opening of a throttle valve in an internal combustion engine is controlled electrically in accordance with a degree of operation of an accelerator by means of a DC motor or the like, and, at the same time, a speed change of an automatic transmission is controlled in accordance with a speed-change map set preliminarily on the basis of a load imposed on the internal combustion engine and a vehicle speed.
In recent years, there have been proposed various kinds of control apparatuses for electrically controlling the opening of a throttle valve in an internal combustion engine by means of a DC motor or the like. In such apparatuses, a throttle command value is calculated in accordance with a degree of accelerator operation or the like, so that the actual throttle opening is controlled by means of a DC motor by using the throttle command value as a target value. Accordingly, a change rate of the throttle opening, that is, a throttle gain, corresponding to a change rate of the degree of accelerator operation can be set arbitrarily in advance. Such apparatuses have an advantage in that they can adapt themselves exactly to a vehicle running state such as an acceleration request. In most cases, the control apparatus of this type have not only a function of controlling the opening of the throttle valve but also a function of controlling the speed changing operation of the automatic transmission. An example of the apparatuses of this type is disclosed in JP-A 1-158255.
The vehicle running control apparatus of this type controls an actual throttle opening in accordance with an accelerator operation degree or the like by means of a DC motor or the like, and, at the same time, performs transmission control by selecting a speed change stage based on a load on the internal combustion engine and a vehicle speed in accordance with a preliminarily set map. At the time of ordinary running, the accelerator operation degree detected by the throttle control is used as a parameter indicating the engine load to thereby select the speed change stage based on the accelerator operation degree and the vehicle speed. At the time of constant speed running control (cruise control), since there is no connection between the accelerator operation degree and the engine load, the speed change stage is selected based on the throttle opening and the vehicle speed.
As described above, at the time of ordinary running of the vehicle, since the conventional vehicle running control apparatus performs transmission control based on the accelerator operation degree and the vehicle speed, speed changing is performed with the similar frequency at every region of the accelerator operation range. However, a driver's request in speed changing varies depending on a region of the accelerator operation degree. In a region where the accelerator operation degree is relatively large, for example, shifting down for acceleration is required, and therefore it is necessary to perform speed changing sensitively in response to a change of the accelerator operation degree. While, in a region where the accelerator operation degree is relatively small, for example, smooth constant speed running or starting of the vehicle is required, and therefore it is necessary to prevent inadvertent speed changing when making a change of the accelerator operation degree. In the case where only the accelerator operation degree is used for transmission control as described above, while transmission control is performed substantially in accordance with the driver's intention in a region where the accelerator operation degree is large, in a region where the accelerator operation degree is small, it has occurred that a speed changing operation is caused by a slight change of the accelerator operation degree, contrary to the driver's intention, thereby making it impossible to perform smooth running and resulting in deteriorated drivability.